COLUMBIA — Republican senators from South Carolina and Georgia have called on the Obama administration to continue funding a program to turn weapons-grade plutonium into commercial nuclear reactor fuel, saying that slowing or ending the project would harm international and domestic relationships.

Sens. Lindsey Graham and Tim Scott of South Carolina and Sens. Saxby Chambliss and Johnny Isakson of Georgia wrote in a letter sent this week to President Barack Obama that efforts to slow funding for the mixed oxide, or MOX, fuel program could hurt both states’ relationships with the federal government.

“Your decision violates the commitments that were made to South Carolina and jeopardizes a 60-year partnership between the Savannah River Site and the state,” the senators wrote. “We will not allow this ill-conceived plan to proceed.”

The MOX plant is under construction at the Savannah River Site, a sprawling complex saddling the South Carolina-Georgia border where nuclear weapons components were once made.

Work at the site, which has long employed thousands of workers from both states, now focuses on research and environmental MOX as part of a nonproliferation effort.

The plant is about 60 percent complete, but the project has undergone years of cost overruns and delays. Last month the Government Accountability Office said the plant is $3 billion over budget, now costing an estimated $7.7 billion. Construction began in 2007, and the GAO also forecast that the MOX facility wouldn’t be oprable until 2019, three years later than originally planned.

On Wednesday, acting U.S. Energy Secretary Daniel Poneman said the administration has been clear that it needs to be a good steward of taxpayer money and see whether there is a better way to honor the disposition agreement. “We are looking at all options, including MOX,” Poneman said.